PNGpong is a dashboard widget that strips the color "correction" information out of the file using pngcrush to fix potential gamma problems. PNGpong then attempts to further compress the PNG file by running it through optipng. Simply drop one or more PNG files onto PNGpong and it'll take care of the rest. (Select PNGs, start to drag, hit F12 and drop the PNGs on the PNGpong widget.)

Great little widget that does both crushing and gamma stripping in one. I also would prefer a standalone app, but this ones still work fine (if you want to, you can always quit dashboard and thus free up memory by using the "stop dashboard" widget).